In the prior art there has been produced a heater comprising an alumina substrate and an electron-conductive pattern provided thereon and designed to generate heat. However, when current is applied through the heater to generate heat, portions near to the cathode terminal (the heat-generating pattern and a portion of the substrate adjacent thereto) become black and increase in electric resistance. In an extreme case, the coating layer is peeled off. Due to the resulting reduction of the service life of the heater, there is a need of applying alternate current or increasing the electric resistance of the heater to limit the current flowing therethrough, thus offering a grave problem in view of use.